1. Field of the Invention
The present invention relates to a solid-state image sensor having an array of photo-sensors, each of which has a set of photosensitive cells different in sensitivity from each other and a microlens converging incident light.
2. Description of the Background Art
As known in the art, a solid-state image sensor of the type is available which actualizes a broader dynamic range for using a primary and a secondary photosensitive cell, which are different in optical aperture area, i.e. photo-sensitivity, from each other. Because the optical aperture area, or opening region, of the primary photosensitive cells is broader, its sensitivity is heightened, and, however, the primary photosensitive cell cannot identify the level of the incident light exceeding a certain intensity level. On the other hand, because the opening region of the secondary photosensitive cell is smaller, its sensitivity is merely lowered, and, however, the secondary photosensitive cell, which does not become saturated as easily as the primary photosensitive cell, can more suitably detect the quantity of the incident light. Moreover, if the output signals of the couple of photosensitive cells are mixed with each other, pixel by pixel, an image signal having broader dynamic range is attained for each of photo-sensors.
Generally, the image signals outputted from the photo-sensors of the solid-state image sensor may involve shading to be irregular in quantity of light incident to the respective photo-sensors due to various causes. For example, in the image sensor type applied to imaging apparatus such as a digital camera, each of the photo-sensors has its own microlens arrayed to cause the incident light to converge so that the optical opening ratio of their photosensitive cells is essentially increased. However, the quantity of light incident to the respective photo-sensors is varies depending upon the incident angle of the light. More specifically, in the vicinity of the edge of a photosensitive array of the image sensor, the light incident to the photo-sensors is often inclined and the direction of the inclination is one-sided specifically to the edge, so that the quantity of the incident light is poorer than one incident to the photo-sensors located near the center of the frame. Consequently, in the vicinity of the edge of a picture picked up, the luminance of produced signals is poorer, thus resulting in shading. It is therefore a common practice with a digital camera of the type described above to use, e.g. digital processing and a memory to correct image signals involving shading, thereby simplifying a shading correcting circuit.
However, in the solid-state image sensor, if a set of photosensitive cells different in sensitivity from each other is arranged in each of photo-sensors corresponding to a particular pixel so that the dynamic range is broadened, the shading of the image signals cannot be adequately compensated for because the photoelectric transduction characteristic differs from one to another photosensitive cell. Co-pending U.S. patent application filed on Jul. 22, 2003 in the name of Naoki KUBO discloses a solid-state image pickup apparatus in which one photosensitive cell with the lower sensitivity of each of the photo-sensors, or pixel positions, is positioned at a side farther from the center of the photosensitive array, e.g. at the top right, top left, bottom right or bottom left portions of the array with respect to the other photosensitive cell with the higher sensitivity of that photo-sensor. The lower-sensitivity photosensitive cells are arranged generally symmetrically with respect to the center of the frame with the higher-sensitivity photosensitive cells intervening toward the center. With this configuration, it is possible to obviate the polarization of shading in the frame and execute the same shading correction on both of the higher- and lower-sensitivity photosensitive cells.
It is a common practice with the solid-state image sensor of the type described above to use, e.g. a solid-state image pickup apparatus such as a digital camera, thereby, with reference to FIG. 3, arranging the photo-sensors to form the photosensitive array 18 of the solid-state image sensor. In FIG. 3, in case an iris device, not shown, which is fitted on the light-incident, or subject, side of the photosensitive array 18, is set to a smaller iris value or the exit pupil position of an optics lens 34 is near to the photosensitive array 18, the light incident to the photosensitive array 18 with inclination is relatively increased. Particularly in the vicinity of the edge of the photosensitive array 18, the incident light is more inclined and more acute toward the edge.
FIG. 5 shows part of the photosensitive array 18 having a plurality of the photo-sensors 12 each corresponding to a particular pixel. Each of the photo-sensors 12 comprises a primary photosensitive cell 22 and a secondary photosensitive cell 24, and a microlens 52 is formed on each photo-sensor 12 to cause luminous flux, or incident light beam, to converge onto the couple of photosensitive cells 22 and 24. In FIG. 5, the light incident to the photosensitive array 18 is refracted by the microlenses 52 in dependence on the incident angle of the light, which makes it difficult that the luminous flux is incident to the photosensitive cells 22 and 24.
For example, with reference to FIG. 8, the light 602 is generally perpendicularly incident to the photosensitive array 18 to successfully converge by microlens 52, thus being incident a sufficiency of the luminous flux to both an opening region 62 of the primary photosensitive cell and an opening region 64 of the secondary photosensitive cell. On the other hand, with reference to FIGS. 9 and 10, the light 604 incident to the photosensitive array 18 is generally inclined to be refracted by the microlens 52. This causes the luminous flux incident to the respective opening regions 62 and 64 of the photosensitive cells 22 and 24 to be decreased so that a convergence ratio of the light is reduced. Particularly in the secondary photosensitive cell 24, because the opening region 64 is smaller, the convergence ratio of the cell 24 is often reduced and the sensitivity is further lowered.
As stated above, in the photosensitive array 18 of the solid-state image sensor 10, reduction in sensitivity of the photo-sensors is apt to occur particularly in the pixel positions where the incident light is inclined, thus involving the shading on the picture picked up. Depending on the incident angle of the luminous flux, the convergence ratio of the photosensitive cells 22 and 24 varies extensively, and therefore, the profile of shading may be varied.
When the incident light forms an image on the photosensitive array 18, a lens 34 of the optics captures the light to cause a blur circle 44, such as image circle, FIG. 4, to appear. If the luminous flux incident to the photosensitive array 18 is inclined as described above, then it converges to be shifted toward the edge direction of the photosensitive array 18. Therefore, with reference to FIG. 4, the blur circles 44 appear on the photosensitive array 18 at the position shifted from the opening regions 42 of the photo-sensors toward the edge direction of the photosensitive array 18. The closer the pixel positions to the edge of the photosensitive array 18, the more the blur circle 44 is shifted from the opening region 42 of the photo-sensor. With the photosensitive array 18 having photo-sensors 12, each of which has the secondary photosensitive cell 24 arranged upward in FIG. 5 with respect to the primary photosensitive cell 22, the blur circle 44 appears, as shown in FIG. 4, thus rendering the convergence ratio of the secondary photosensitive cells 24 lowered particularly in the pixel positions near the lower part of the photosensitive array 18.
FIG. 9 shows one of the photo-sensors 12 positioned near the upper portion of the photosensitive array or imaging frame 18. In the photo-sensor 12, if the more incident light 604 is inclined, the less luminous flux is incident to the respective opening regions 62 and 64 of the photosensitive cells 22 and 24. However, the opening region 62 of the primary photosensitive cell 22 has larger photosensitive area, and the opening region 64 of the secondary photosensitive cell 24 is positioned near the edge of the photosensitive array-18, i.e. on the side at which the light is captured, so that the luminous flux is relatively efficiently captured. By contrast, FIG. 10 shows one of the photo-sensors 12 positioned near the lower part of the imaging frame 18, wherein the luminous flux converging by microlens 52 is easily incident to the opening region 62 of the primary photosensitive cell 22. However, this luminous flux converging by microlens 52 is difficult, and may not, be incident to the opening region 64 of the secondary photosensitive cell 24 because the incident angle is acute, thus reducing often the convergence ratio. Consequently, the quantity of the lights incident to the secondary photosensitive cells 24 thus differs between the upper and lower portions of the photosensitive array 18, thereby often involving the luminance shading which is asymmetrical on the upper and lower portions of a picture picked up.